FIG. 8 shows a typical configuration of a thermal shut-down circuit.
Thermal shut-down circuit 100 is connected to a specific circuit 200 and can stop the operation of circuit 200 when the temperature rises. Said circuit 200 shown in the figure is driven by a bias current supplied from constant current source 201 when a power supply voltage VDD is supplied to constant current source 201.
Thermal shut-down circuit 100 comprises constant current source 101 and Zener diode ZD connected in series between power supply voltage VDD and reference potential VSS, resistors R100 and R101 connected in series between the cathode of Zener diode ZD and reference potential VSS, and NPN transistor Q100 used as a power control switch with its base connected to the node between resistors R100 and R101, its collector connected to the power end of circuit 200, and its emitter held at reference potential VSS.
When power supply voltage VDD is supplied, a reference current is supplied from constant current source 101 and flows toward resistors R100 and R101. At this time, since the cathode of Zener diode ZD is kept at a constant reference voltage VREF, the base bias voltage of NPN transistor Q100 is determined by the constant voltage at the node between resistors R100 and R101.
Since the base-emitter voltage Vbe of NPN transistor Q100 has a temperature characteristic of about −2 mV/° C., the base bias voltage of NPN transistor Q100 increases along with a rise in temperature. When a prescribed temperature is reached, NPN transistor Q100 is turned on. When NPN transistor Q100 is on, the potential at the power end of circuit 200 is pulled back to reference potential VSS, and thus the current supplied to circuit 200 is shut down. Circuit 200 is protected or its output is stopped by this thermal shut-down operation.
Said thermal shut-down circuit 100 has the following problems.
First of all, the detected temperature varies significantly depending on the temperature characteristic of Zener diode ZD at the reference voltage.
Second of all, the detected temperature varies significantly depending on changes in the characteristics of NPN transistor Q100 and the variation occurring during manufacturing.
Third of all, since NPN transistor Q100 gradually enters into the on state from the off state as the temperature rises, a sharp thermal shut-down characteristic cannot be obtained.
Since said thermal shut-down circuit 100 might operate at a temperature out of the range of reliable circuit operation when the aforementioned problems occur at the same time, it is difficult to guarantee a temperature range for reliable operation. Also, the wide range of detection temperatures of thermal shut-down circuit 100 is a serious problem.